This invention relates in general to electronic fluorescent display devices and in particular, to an improved low voltage cathodoluminescent device particularly useful for full color hang-on-wall type displays.
Researchers in many flat panel display technologies, such as LCD, PDP, EL, LED, VFD, flat CRT, have been trying to develop a full-color hang-on-wall television. Color televisions of several inch to ten inch screens using LCD technology have been produced. Such televisions using LCD employ a large number of thin film transistors on their basic boards and are expensive. Because of difficulty of manufacture, it is difficult to further increase the size of the basic board and of the television screen of such products. LCD televisions employ a back illumination scheme. The basic board with thin film transistors transmits a low proportion of light from a light source and this limits the brightness of the display. Because of these difficulties, in order to develop larger color televisions using LCD technology, research in this area is primarily focused on projection televisions.
Color televisions using PDP technology is still in the research stage and at this point, color televisions of twenty inch screen have been proposed. The main problems in the development of PDP type color televisions include its low efficiency in phosphorescence, its complicated drive circuitry, unevenness in brightness and short product life. Research in LED, EL still has not been able to develop luminescent elements for blue lights. While multi-color displays have been developed using VFD, such devices are limited to smaller television screens. Furthermore, aside from the use of luminescent elements using zinc oxide and zinc for generating blue-green light, the brightness, efficiency and product life of other color phosphors are still not satisfactory. From the above, it will be evident that large-screen flat full-color hang-on-wall televisions that have been proposed using any of the existing flat panel display technologies are not entirely satisfactory.
Cathode ray tubes (CRT) have been used for display purposes in general, such as in conventional television systems. The conventional CRT systems are bulky primarily because depth is necessary for an electron gun and an electron deflection system. In many applications, it is preferable to use flat display systems in which the bulk of the display is reduced. In U.S. Pat. No. 3,935,500 to Oess et al., for example, a flat CRT system is proposed where a deflection control structure is employed between a number of cathodes and anodes. The structure has a number of holes through which electron beams may pass with sets of X-Y deflection electrodes associated with each hole. The deflection control structure defined by Oess et al. is commonly known as a mesh-type structure. While the mesh-type structure is easy to manufacture, such structures are expensive to make, particularly in the case of large structures.
Another conventional flat panel system currently used is known as the Jumbotron such as that described in Japanese Patent Publication Nos. 62-150638 and 62-52846. The structure of Jumbotron is somewhat similar to the flat matrix CRT described above. Each anode in the Jumbotron includes less than 20 pixels so that it is difficult to construct a high phosphor dot density type display system using the Jumbotron structure.
Both the flat matrix CRT and Jumbotron structures are somewhat similar in principle to the flat CRT system described by Oess et al. discussed above. These structures amount to no more than enclosing a number of individually controlled electron guns within a panel, each gun equipped with its own grid electrodes for controlling the X-Y addressing and/or brightness of the display. In the above-described CRT devices, the control grid electrodes used are in the form of mesh structures. These mesh structures are typically constructed using photo-etching by etching holes in a conductive plate. The electron beams originating from the cathodes of the electron guns then pass through these holes in the mesh structure to reach a phosphor material at the anodes. As noted above, mesh structures are expensive to manufacture and it is difficult to construct large mesh structures. For this reason, each cathode has its own dedicated mesh structure for controlling the electron beam originating from the cathode. Since the electron beam must go through the hole in the mesh structure, a large number of electrons originating from the cathode will travel not through the hole, but lost to the solid part of the structure to become grid current so that only a small portion of the electrons will be able to escape through the hole and reach the phosphor material at the anode. For this reason the osmotic coefficient, defined as the ratio of the area of the hole to the area of the mesh structure of the cathode, of the above-described devices is quite low.
As taught in the parent application, to avoid the problem of low osmotic coefficient in conventional devices, instead of using individually controlled electron guns, two or more sets of elongated grid electrodes may be employed for scanning and controlling the brightness of pixels at the entire anode where the area of the grid electrodes that blocks electrons is much smaller than the area of the mesh structure of the conventional devices.
The above-described CRT devices have another drawback. In the case of the Jumbotron, each electron gun is used for scanning a total of 20 pixels. In the Oess et al. patent referenced above, each electron beam passing through a hole is also used for addressing and illuminating a large number of pixels. When illumination at a particular pixel is desired, certain voltages are applied to the X-Y deflection electrodes on the inside surface of the hole, causing electrons in the electron beam passing through the hole to impinge the anode at such pixel. However, electrical noise and other environmental factors may cause the electron beam in the Oess et al. system and the Jumbotron to deviate from its intended path. Furthermore, certain electrons will inevitably stray from the electron beam and land in areas of the anode which is different from the pixel that is addressed. This causes pixels adjacent to the pixel which is addressed to become luminescent, causing crosstalk and degrades the performance of the display.
As is known to those skilled in the art, the inner chamber of a cathodoluminescent visual display device must be evacuated so that the electrons emitted by the cathode would not be hindered by air particles and are free to reach phosphor elements at the anode. For this reason, the housing for housing the cathode, anode and control electrodes must be strong enough to withstand atmospheric pressure when the chamber within the housing is evacuated. When the display device has a large surface area, as in large screen displays, the force exerted by the atmosphere on the housing can be substantial when the chamber within the housing is evacuated. For this reason, conventional cathodoluminescent display devices have employed thick face and back plates to make a sturdy housing. Such thick plates cause the housing to be heavy and thick so that the device is heavy, and expensive and difficult to manufacture. It is therefore desirable to provide an improved cathodoluminescent visual display device where the above-described difficulties are not present.